Low-distortion programmable-gain amplifiers have many applications. For example they are useful in processing analog audio signals where it is important to preserve the integrity of the signals. One prior art implementation of a low-distortion programmable-gain amplifier is shown in FIG. 1. An input signal, which may be AC or DC, is applied to the input VIN. The output signal appears at the output VOUT. In this embodiment the high-gain operational amplifier A1 is configured as a non-inverting amplifier. This configuration is preferred for low-noise applications over the inverting configuration since the feedback network can be made low impedance to minimize its thermal noise contribution without compromising the amplifier input impedance, which may be set independently via resistor RIN. The feedback network around operational amplifier A1 is tapped at any one of a plurality of points by selectively controlling the corresponding electronic switch elements S1 through SN. These switch elements are typically each constructed of complementary metal-oxide semiconductor (CMOS) devices. Control signals (C1 through CN) select the desired gain by turning on the associated switch. Such an approach has the benefit that the variations in ON-resistance of electronic switches S1 through SN due to changes in input voltage do not affect the linearity of the output signal since no signal current flows through these switches. This minimizes distortion, so long as one and only one of electronic switches S1 through SN is turned on at any one moment in time.
However, the ON-resistance of each of these switches does contribute thermal noise to the total input noise of the amplifier. One way to decrease the ON-resistance of CMOS electronic switches (and thus to reduce the amplifier's input noise) is to increase the physical width of the CMOS devices which make up the switches. In an integrated circuit, however, an increase in the width of a switch results in an increased die area. Since the approach illustrated in FIG. 1 requires one switch for each desired gain setting, the necessary area taken by the switches in an integrated circuit can be a significant issue.
Another aspect of CMOS electronic switches is that modern CMOS processes often do not allow large voltages to be applied between the gate and channel (source and drain) of the switches, even for so-called “high-voltage” CMOS processes. This can limit the analog voltages which may be switched by CMOS electronic switches, thus restricting the maximum analog voltages, Vin, which can be applied at input of the switch.
Accordingly, it is desirable to provide a dynamic switch driver for a low-distortion programmable-gain amplifier that overcomes or at least substantially reduces the foregoing disadvantages.